Datacenter physical infrastructure (2/4): the generators

Contents

In the first article we mapped the complete power chain, from the utility feed to the rack, and left pending the links that decide resilience against a power cut. We start with the one that carries the weight of the hours: the generator set. The UPS, which we will see at the end of the series, is a sprinter that covers seconds; the generator is a long-distance runner, capable of sustaining the entire facility for hours or days while the utility fails to come back. And in the AI era it has stopped being a mere insurance policy to become, at times, the main source of power.

The generator’s role: long-duration backup

A datacenter’s line of defence against a grid failure is clear: the UPS takes over instantly, with no perceptible interruption, and sustains the load during the few seconds the generator needs to start, stabilise and accept load. From there on, the generator takes over the work and the UPS returns to its role as conditioner and reserve. The UPS saves the gap; the generator saves the outage. For a datacenter, units of 1 to 3 MW are typically used, sized to support 100 % of the load for the whole duration of the event, not to give a brief push.

What has changed in 2025-2026 is the purpose. The AI boom, combined with the impossibility of getting a grid connection within reasonable timescales, has pushed many projects to use on-site generation as the primary source (behind-the-meter), not as an emergency supply. Some 92 % of announced behind-the-meter projects (around 82 GW) date from the start of 2025 onwards. The extreme case is the off-grid Oracle and OpenAI datacenter in Texas: a 700 MW natural gas microgrid with almost two hundred Jenbacher engines dedicated to prime power and a dozen more for backup, to feed up to 1.4 GW of compute. The generator, which for decades was the actor waiting in the shadows, has walked onto the main stage. This change of role has cascading consequences, in the rating that has to be specified, in the applicable emissions regime, in fuel logistics and in maintenance, which run through this entire article. It is worth keeping in mind: many of the design decisions depend on whether the generator is an insurance policy that starts once a year or a power station that runs for thousands of hours.

Starting in 10 seconds

The requirement that defines the design of a mission-critical generator is the starting time. NFPA 110, which governs emergency power systems, classifies installations by level and by type. Datacenters usually use Level 1, Type 10: the system must deliver acceptable power to the loads within 10 seconds of the grid failure, and at Level 1 all loads are transferred within that time, regardless of the size of the facility. Ten seconds is, precisely, the gap the UPS has to cover with its batteries.

But starting on time is only half the challenge. The other half is accepting the load without destabilising. When a newly started generator receives a large block of load all at once, the rectifier bank of a UPS for example, the alternator voltage and the engine frequency dip momentarily (voltage dip, frequency dip) before recovering. If those dips are excessive, the server power supplies can trip and reboot in a loop even though the generator is running: the power cut has been dodged, but the system goes down anyway through a badly managed transient. This is the difference between a generator that “starts” and one that “sustains”.

Here is one of the technical reasons why diesel still dominates over gas. Diesel injects the fuel directly into the cylinder and can accept load blocks close to 100 % with tolerable dips. The gas engine, by contrast, opens an intake valve and the air-fuel mixture must travel through the manifold before burning, which introduces a delay: a turbocharged gas engine can accept steps as low as 10-15 %, against 60-80 % for diesel. For a sensitive load that appears all at once, that difference is decisive.

The framework that quantifies all this is the ISO 8528-5 standard, which defines performance classes G1 to G4 with increasingly strict limits on voltage and frequency deviation. For sensitive electronic loads (UPS, drives) the reference class is G3, which requires voltage regulation around ±1 % and frequency regulation of ±0.25 % in steady state, with fast recovery from transients. Class G4, stricter still, is agreed between manufacturer and customer. It is worth remembering that UPS units are especially sensitive to the rate of change of frequency: variations above half a hertz per second can make them reject the source.

The ratings: why “standby” is not enough

Not all generators of the same nominal power are equivalent; what distinguishes them is their rating, which defines how many hours and at what load factor they can work. ISO 8528-1 defines four:

  • ESP (Emergency Standby Power): variable load, up to about 200 hours a year, with an average load factor not exceeding 70 % over 24 hours. It is the classic “emergency backup”.
  • PRP (Prime Power): variable load, unlimited hours, average load factor around 70 %.
  • LTP (Limited-Time Power): up to about 500 hours a year.
  • COP (Continuous Operating Power): unlimited hours at constant load, normally at 100 % indefinitely.

To those four the industry has added a commercial rating specific to our world: DCC (Data Center Continuous). It is not part of ISO 8528-1, being manufacturer nomenclature (Cummins, Caterpillar/MTU), but it has been reviewed by the Uptime Institute and is considered suitable for Tier III and IV. Its distinguishing feature is that it allows unlimited hours with no average load factor restriction, neither variable nor constant. Why does this matter? Because a datacenter operating behind-the-meter, doing peak shaving, or simply wanting Uptime certification without the hour and load limitations of the Standby rating cannot settle for an ESP. The choice of rating is not a catalogue detail: it determines whether the generator is legally and technically fit for the use it is going to be put to.

Sizing it right: the devil in the transients

As we saw in the first article, generators are sized in apparent power (kVA), applying the power factor, with 0.8 the industry standard for mixed loads:

$$S_{\text{kVA}} = \frac{P_{\text{kW}}}{\text{PF}}$$

But the real sizing of a generator is rarely set by the steady-state load; it is set by the transients and the problem loads. Three factors force oversizing:

First, motor starting. Induction motors demand five to seven times their rated current on direct starting, with a very low power factor (0.2-0.35), which can translate into a peak of 2.5 to 3.5 times the motor’s rated kVA. Cooling pumps and compressors starting at the same time can sink the frequency of a badly sized generator. Soft starters and drives reduce that blow.

Second, non-linear loads and harmonics. UPS units and drives inject harmonic distortion (THD) that heats the alternator and degrades the waveform. The rule of thumb is to reduce the available capacity by around 0.8 % for each point of THD above 5 %, which in practice can require oversizing by between 15 and 25 %, or specifying an alternator with 2/3 pitch winding and permanent magnet excitation (PMG), which is more tolerant of distortion.

Third, derating for ambient conditions: above 40 °C of temperature or 1000 m of altitude, the generator loses capacity and this must be corrected. Adding it all up, the manufacturers and NFPA 110 itself recommend a safety margin of the order of 20-25 %. Sizing a generator “exactly” for the nominal load is the recipe for the cooling not starting on the day of the outage.

It is worth seeing the combined effect in a case. Returning to the example from the first article, a row that added up to the order of 1350 kW of total load with its cooling, in steady state that called for some 1690 kVA of generation. But if the cooling of that row includes several pumps and compressors starting at the same time after the outage, the transient starting peak can momentarily demand considerably more than those 1690 kVA, and the alternator must hold the frequency during that instant without the electronic loads dropping out. It is common that, after adding the margin for motor starting, the harmonic tolerance of the UPS and the ambient derating, the set selected for a row “of 1 MW of IT” ends up at 2 to 2.5 MVA, and that two are installed in N+1. A generator is not sized by what the load consumes, but by the worst instant it has to survive.

Diesel, gas, hydrogen and batteries

The choice of generation technology is one of the great debates of 2025-2026, and it pays to understand why, despite all the alternatives, diesel keeps winning.

The figure for the “diesel boom” is eloquent: installed diesel generation capacity in United States datacenters went from some 20 GW in 2018 to 55 GW in 2024, nearly tripling; in Virginia alone more than 10,500 units, 27 GW, had been permitted by the end of 2025. The reason is not nostalgia but fit. An AI datacenter is commissioned in 18-24 months, while a new gas plant depends on turbines with delivery times of 2 to 4 years and on grid connection queues that reach 7 years. Diesel is the only option that matches the speed, the risk tolerance and the regulatory framework of AI deployment, and it also accepts large load blocks and does not depend on a pipeline. Cummins has delivered more than 39 GW of equipment to datacenters and doubled its production capacity in 2025.

The alternatives are advancing, but slowly. Natural gas, with its greater autonomy, is the basis of the large behind-the-meter prime power projects, at the cost of a slower transient response. Bi-fuel systems combine diesel and gas to extend autonomy, reverting to diesel if the gas fails. Fuel cells are gaining traction: Bloom Energy (solid oxide) has signed agreements with hyperscalers and is moving from backup to prime power, and Microsoft tested with Caterpillar a 3 MW hydrogen fuel cell capable of feeding a facility for 48 hours, with the goal of eliminating diesel by 2030. Batteries (BESS), for their part, are being integrated into medium-voltage architectures to cover short events and reduce generator running hours, with the generator only starting on extended disturbances. But diesel remains the reference standard, and the alternatives coexist with it more than they replace it.

The table sums up the trade-offs each technology settles:

TechnologyTransient responseAutonomyEmissions in useMaturity 2026
DieselExcellent (blocks ~100 %)Limited by tank (24-72 h)High; requires Tier 4 / SCR+DPFDominant
Natural gasSlow (low steps)High (depends on pipeline)Lower than dieselGrowing (prime / behind-the-meter)
Bi-fuelGood (reverts to diesel)HighIntermediateNiche
Fuel cell (H₂/SOFC)VariableDepends on H₂/gas supplyVery low or noneEmerging
BESS (batteries)InstantaneousShort (minutes to a few hours)None in useComplementary

The reading for an architect is that there is no “best” technology, but rather a combination appropriate to each case: diesel provides the response and the supply independence that critical starting demands, gas and fuel cells provide autonomy and low emissions for prolonged operation, and batteries cut engine hours. The most advanced architectures of 2025-2026 are hybrid for precisely that reason.

Emissions: the tension with regulation

Burning diesel at gigawatt scale has an environmental cost that regulation watches closely, and here there is a tension every AI architect must know about. The United States EPA Tier 4 Final standard for diesel engines requires drastic reductions in nitrogen oxides and particulates, of the order of 93-94 % against the first generations, which forces full aftertreatment with a particulate filter (DPF) and selective catalytic reduction (SCR) with urea (DEF). Meeting Tier 4 Final is expensive and bulky.

The critical nuance is that strictly emergency engines are exempt from the most severe requirements: they can run up to about 100 hours a year for testing and maintenance (of which a maximum of 50 for non-strictly-emergency uses), with no time limit during a real emergency. That framework fits perfectly with the traditional role of the generator as backup. But it collides head-on with behind-the-meter use as a primary source, which means thousands of hours a year and requires far more demanding environmental permits.

This tension is one of the knots of AI deployment. A generator classified as “emergency” is cheap to permit and to maintain, but it cannot be the main source without breaking the framework that grants it that exemption; a prime power generator is legally fit to run for thousands of hours, but it carries the cost and the bulk of full Tier 4 aftertreatment and environmental permits that are much slower to obtain. Where the rush to deploy compute collides with local air quality, projects fall back on “minor source” permits and on spreading the load across many small units, an exercise in regulatory as much as electrical engineering. And social pressure is rising: communities near large datacenters look ever more closely at the megawatts of diesel installed beside them. The industry navigates that tension with “minor source” permits and with architectures that spread the load, but it is regulatory ground in full evolution. In Europe, the equivalent standard is Stage V, which also requires aftertreatment for large engines. And a decarbonisation route with no retrofit is emerging: HVO (renewable diesel of vegetable origin), a drop-in substitute that reduces CO₂ by up to 90 %, is stored for up to ten years with low microbial risk, and which the main manufacturers already certify for their sets.

The fuel: autonomy and degradation

A generator is worth no more than its fuel supply. The prevailing design in large datacenters combines a belly tank integrated under each set with refuelling by tanker truck. The recommended autonomy goes from 12-24 hours per generator in normal facilities to 48 or 72 hours in mission-critical ones, and more still in seismic or remote areas.

The autonomy calculation is direct: the time the generator holds out is the fuel volume divided by the consumption,

$$t_{\text{autonomy}} = \frac{V_{\text{tank}}}{\dot{m}_{\text{consumption}}}$$

and consumption follows a useful rule of thumb: about 90 gallons per hour per MW at full load, equivalent to approximately 0.1 gallons of diesel per kWh generated. A 2 MW set can consume of the order of 140 gallons per hour at full load, so 72 hours at 75 % load requires of the order of 7,500 gallons in store. Multiply by an N+1 fleet of several sets and you understand why fuel logistics is a project in itself.

And there is a trap that gets forgotten: diesel degrades. It starts to oxidise at 6-12 months, accumulates water through condensation and encourages microbial growth that generates sludge and acids capable of clogging filters and injectors. The solution is polishing, filtering the stored fuel without emptying the tank, every 3-6 months, and not storing diesel for more than a year without treatment. A generator that will not start on the day of the outage because its fuel has spoiled is as absolute a failure as not having one at all.

The fuel architecture usually has two levels: a main tank (bulk, or the belly tank under the set) with the total autonomy, and a smaller day tank next to the engine that refills automatically and guarantees immediate, stable supply. Redundancy of supply, with refuelling contracts for tankers and priority in the face of a regional disaster affecting many customers at once, is part of the resilience design, not a logistical patch. And here the transition to HVO changes the equation for the better: besides cutting CO₂ by up to 90 % with no engine modification, it is far more stable in storage, holding up for as much as ten years with low microbial risk, which relieves precisely the degradation problem of fossil diesel. It is no coincidence that the major manufacturers already certify their sets for HVO and that several European operators have adopted it.

Paralleling: several sets as one

A serious facility rarely depends on a single generator. N+1 redundancy requires several sets working in parallel, and coordinating them is the job of the paralleling switchgear. Putting two generators in parallel is not trivial: before closing the breaker that joins them, three parameters must match, namely voltage, frequency and, above all, phase angle, because closing out of synchronism can cause severe mechanical damage. Digital controls (Cummins PowerCommand, Caterpillar EMCP, Woodward) decide the safe instant to close.

Once in parallel, the system shares the load proportionally among the sets (load sharing), both active and reactive power. In island operation, the case of a power cut, isochronous control keeps the frequency constant; when paralleling with the grid, droop control is used. And then the magic of redundancy: when a running set fails, the controls start, synchronise and bring in the standby one, rebalancing the load in seconds. The cost of all this capability is not small: a turnkey diesel system at large scale moves in market ranges of the order of 650,000 to more than 1,000,000 USD per MW, and the paralleling switchgear for a hyperscaler can add up to several million.

The choice between N+1 and more robust configurations reappears here with a nuance specific to generation. An N+1 generator plant, four sets for a load that needs three, for example, tolerates the failure of one set without losing the facility, and it is the usual configuration for its cost-reliability balance. But it pays to look at where the single point of failure is: if all the sets feed a single bus through one paralleling switchgear lineup, that board becomes a critical link that must be designed with its own redundancy or segmentation. Replicating generators without replicating the bus that joins them is solving half the problem. In fault-tolerant facilities, the logic of the two A/B paths we saw in the first article extends to generation as well: two independent plants, each feeding its own path, with no common point where a single failure brings everything down.

Maintenance and testing: the enemy is low load

It ends where the first article ended: reliability is earned by rehearsing. And with diesel generators there is a specific and counter-intuitive enemy: low load. Running a diesel for a long time below its optimal load causes wet stacking, the accumulation of unburnt fuel in the exhaust, which reduces efficiency and can damage the engine. That is why testing does not consist merely of starting the set, but of loading it for real.

The instrument is the load bank, which applies real load to the generator to burn off those residues and, along the way, reveal latent faults in cooling, injectors or voltage regulation that a no-load start would never show. NFPA 110 sets the rhythm: a monthly test of the diesel at a minimum of 30 % of its rated power, and an annual load bank protocol applying 50 % for half an hour followed by 75 % for an hour. Skipping these tests is building paper resilience: the generator appears to be there, but nobody has checked that it will sustain the load when it really matters.

There is a paradox in all this worth bearing in mind. The generator is probably the most tested and least used machine in a well-run datacenter: it spends 99.9 % of its life switched off, waiting for an event that one hopes never arrives, and yet it is rehearsed religiously every month. That discipline is not bureaucracy: it is the only way to be certain that, in the seconds when the grid blinks and the UPS drains its batteries, the handover happens. A generator that is never tested under load is not a backup, it is an expensive assumption. The reliability of the electrical layer, like that of any critical system, is built with redundancy and confirmed by rehearsal.

Key takeaways

The generator is the long-distance runner of the power chain: it starts in ten seconds, accepts load blocks without destabilising, and sustains the facility for hours or days with a rating, ideally Continuous or DCC, matched to its real use. Sizing it right means thinking about transients and harmonics, not just the nominal load; feeding it right requires fuel logistics with autonomy and polishing; and relying on it requires redundant paralleling and load bank testing. In the AI era, moreover, it has crossed a boundary: from emergency insurance to, increasingly, primary source of power, with all that implies for emissions and regulation.

But the generator does not act alone. Between the grid, the generator and the load there is a referee that decides at every instant where the power comes from: the switching. The third article of the series, which we will publish next, will go into ATS and STS units, and into how the load is transferred between sources without the compute noticing.

See also

Sources